1. Field of the Invention
The present invention relates to a zoom lens (a zoom lens barrel), and more specifically to a mechanism for guiding more than one lens frame in the direction of the optical axis of the zoom lens.
2. Description of the Related Art
It is known in the art for a zoom lens, or a zoom lens barrel, having a mechanism which guides a plurality of lens frames (lens groups) in the optical axis direction of the zoom lens without rotating about the optical axis, so that the plurality of lens frames are driven in the optical axis direction in a predetermined moving manner by rotation of a cam barrel having cam grooves. In such a conventional lens guiding mechanism, the plurality of lens frames are individually guided along the optical axis without rotating about the optical axis, while the plurality of lens frames are driven in the optical axis direction in a predetermined moving manner in accordance with the contours of the cam grooves of the cam barrel. Such a conventional lens frame guiding mechanism has no substantial adverse effects if it is used for the zoom lens of a conventional camera using light-sensitive film such as 35 mm or APS compact zoom camera.
However, such a conventional lens frame guiding mechanism does have substantial adverse effects if it is used for the zoom lens of a digital camera because object images are formed on the sensitive surface of a small CCD (CCD image sensor) which is much smaller than the picture plane of conventional cameras using light sensitive film. Namely, upon assembly, every lens element of a digital camera must be optically centered, correctly spaced, and held firmly with a relatively high precision, e.g., tens times greater than that required in conventional cameras using light-sensitive film. For instance, if the angle of view is constant, the focal length of a photographing lens becomes shorter as the size of the picture plane reduces, which in turn reduces the sizes of all the elements of the photographing lens such as, e.g., lens elements, lens frames. Therefore, the influence that a tolerance (e.g., 10 xcexcm) has on a photographing lens system of a digital camera is much larger than the influence that the same tolerance would have on a photographing lens system of a conventional camera using light-sensitive film. Accordingly, manufacturing or assembling error which falls within tolerance of optical performance in the photographing optical system of a conventional camera using light-sensitive film can be outside the tolerance of optical performance in the photographing optical system of a digital camera. Specifically in a photographing lens system of a digital camera, the influence that eccentricity or tilt of one or more lens frames (one or more lens groups) relative to another lens frame or frames (lens group or groups) has upon the optical performance of the photographing optical system is larger than the influence of deviation of one or more lens frames (one or more lens groups) in the optical axis direction relative to another lens frame or frames (another lens group or groups).
From this point of view, in the conventional lens frame guiding mechanism, a manufacturing or assembling error of each element of the zoom lens tends to exert adverse effects upon the optical performance of the photographing optical system since a plurality of lens frames are independently guided in the optical axis direction without rotating about the optical axis. For instance, if there is positional error such as eccentricity in each of the lens frames, the linear guide member, and the cam barrel, there is a possibility of these errors being accumulated to deteriorate the optical performance of the photographing optical system, especially in the case of eccentricity of one or more lens groups relative to another lens group or groups.
The present invention has been device in view of the above-mentioned problems; accordingly, an object of the present invention is to provide a lens frame guiding mechanism, used in a zoom lens, which guides a plurality of lens frames in the optical axis direction of the zoom lens without rotating about the optical axis so that the plurality of lens frames are driven in the optical axis direction by rotation of a cam barrel having cam grooves, wherein eccentricity of one or more lens groups relative to another lens group or groups does not occur easily.
To achieve the object mentioned above, according to an aspect of the present invention, a lens frame guiding mechanism of a zoom lens is provided, including a plurality of lens frames each having a plurality of projections which extend radially outwards at predetermined intervals in a circumferential direction, wherein a cam follower is formed on each of the projections; a linear guide barrel provided around the plurality of lens frames, wherein the linear guide barrel is guided along the optical axis of the zoom lens without rotating; a plurality of linear guide slots formed on the linear guide barrel to extend parallel to the optical axis, wherein the number of the plurality of linear guide slots corresponds to the number of the plurality of projections of each of the lens frames so that the plurality of projections of each of the lens frames are respectively engaged in the plurality of linear guide slots to be slidable in a direction of the optical axis; a cam barrel fitted outside the linear guide barrel to be rotatable about the optical axis relative to the linear guide barrel; and a plurality of bottomed cam grooves formed on an inner peripheral surface of the cam barrel for each of the plurality of lens frames, in which the plurality of cam followers of a corresponding lens frame of the plurality of lens frames are respectively engaged, wherein the profiles of the cam grooves for one of the plurality of lens frames is different from the profiles of the cam grooves for the other of the plurality of lens frames. The plurality of lens frames are moved in the direction of the optical axis, without rotating about the optical axis, to change a focal length of the zoom lens via rotation of the cam barrel.
Preferably, the number of projections of each of the lens frames, the number of the linear guide slots of the linear guide barrel, and the number of the bottomed cam grooves of the cam barrel are three, respectively arranged at 120xc2x0 equi-angular intervals.
Preferably, the cam barrel is fitted on the linear guide barrel so as to be immovable in the direction of the optical axis relative to the linear guide barrel.
Preferably, each of the plurality of projections is formed to have opposite faces extending parallel to each other so that the opposite faces are respectively in sliding contact with side faces of a corresponding linear guide slot of the plurality of linear guide slots.
In an embodiment, the zoom lens is incorporated in a digital camera.
According to another aspect of the present invention, a lens frame guiding mechanism of a zoom lens is provided, including a first lens frame having a first group of three projections extending radially outwards at equi-angular intervals, each of the first group of three projections having a first cam follower; a second lens frame having a second group of three projections extending radially outwards at equi-angular intervals, each of the second group three projections having a second cam follower; a linear guide barrel provided around the first lens frame and the second lens frame, and guided in a direction of the optical axis without rotating about the optical axis; three linear guide slots formed on the linear guide barrel so as to extend parallel to the optical axis, the first group of three projections and the second group of three projections being slidably engaged in the three linear guide slots, respectively; a cam barrel fitted on the linear guide barrel to be rotatable about the optical axis relative to the linear guide barrel; a first group of three cam grooves which are formed on an inner peripheral surface of the cam barrel so that the three first cam followers are respectively engaged in the first three cam grooves; and a second group of three cam grooves which are formed on the inner peripheral surface of the cam barrel so that the three second cam followers are respectively engaged in the second group of three cam grooves. The first and second lens frames are moved in the direction of the optical axis, without rotating about the optical axis, in a predetermined moving manner to change a focal length of the zoom lens by rotation of the cam barrel.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-24039 (filed on Feb. 1, 2000) which is expressly incorporated herein by reference in its entirety.